In recent years, locking and unlocking of vehicle doors at a distance by means of a portable device carried by drivers are generally practiced in addition to locking and unlocking vehicle doors directly with a key. Along with this trend, an antenna device and a communications system using the antenna device for enabling secure communication with a portable device are being demanded.
Japanese Translation of PCT Publication No. 2001-520337 discloses a configuration of an antenna device in a conventional communications system in which an antenna is disposed in the vicinity of the opening/closing section, for example, inside a doorknob.
Referring to FIG. 5 and FIG. 6, a description will be given on this conventional antenna device and a communications system using it.
FIG. 5 is a perspective view of a conventional antenna device. As shown in FIG. 5, antenna 5 of the conventional antenna device is disposed inside knob 2 made of resin provided on door 1 which is made of a metallic chassis such as a steel plate. And, knob 2 is provided in a manner such that its right end is swingable relative to door 1. Also, antenna 5 is formed by winding a coil on the periphery of a rod-like core of a magnetic material. Antenna 5 is housed in a hollow space inside knob 2 in a manner such that its axis line is parallel to door 1.
FIG. 6 is a diagram to show an example of a conventional antenna device configuration and a communications system that uses the antenna device. FIG. 6 also shows an example of directivity characteristics of a conventional antenna device. As shown in FIG. 6, antenna 5 is connected to control circuit 4 provided on a vehicle. The communications system is so configured that control circuit 4 communicates with a portable device (not shown) carried by a driver through antenna 5.
In the above configuration, when a driver carrying a portable device approaches the vehicle and enters the region of communication range D, control circuit 4 communicates with the portable device through antenna 5. On identifying the driver, control circuit 4 drives a locking mechanism (not drawn) such as a solenoid and door 1 is unlocked.
Also, when the driver leaves the vehicle to outside of the region of communication range D, control circuit 4 becomes unable to communicate with the portable device. The system is so configured that, in such a case, conversely to what is described above, control circuit 4 drives the locking mechanism and locks door 1.
In such a conventional communications system, antenna 5 is housed inside knob 2 on the front right and left sides of a vehicle in a manner such that its axis line is parallel to door 1. As a result, antenna 5 radiates electromagnetic waves as shown by an example of antenna directivity characteristics of FIG. 6. Accordingly, the communication range of a conventional communications system is as represented by communication range D due to the electromagnetic waves radiated outwardly of vehicle 6.
That is, as the inner metallic door 1 is overwhelmingly large compared with small antenna 5 disposed with its axis line parallel to door 1, the inward magnetic field of antenna 5 makes a magnetic path via door 1 and the inwardly radiated electromagnetic waves are absorbed by door 1. Accordingly, the electromagnetic waves are radiated from antenna 5 only outwardly to communication range D.
For the reason described above, when a driver is in the vehicle cabin, communication with a portable device carried by the driver is not possible with antenna 5 alone that is housed inside knob 2. Accordingly, in a conventional communications system, another antenna (not drawn) connected to control circuit 4 is usually provided in the cabin to enable communication with a portable device.
Also, as antenna 5 is close to metallic door 1 and radiated electromagnetic waves are absorbed by door 1, radiation efficiency of antenna 5 is low, requiring an antenna current of the order of 1.0 A in order to secure outward communication range of about 1 m in radius, thus resulting in a relatively large consumption of electric power.
As has been described, with the above conventional antenna device and a communications system using the antenna device, a separate antenna was necessary in order to communicate with a portable device in the vehicle cabin. Accordingly, the configuration of the antenna device suffers a problem of a complicated structure and expensive cost.